18 TH INTERNATIONAL CONFERENCE ON COMPOSITE MATERIALS ELECTRICAL AND OPTICAL PROPERTIES OF COMPOSITE TRANSPARENT CONDUCTING OXIDE THIN FILMS K.S. Hui 1 , Lei Li 2 , J.Y. Lee 2 , H.W. Park 2 , P.K. Song 2 , Y.R. Cho 2 , H.S. Lee 2 , K.N. Hui 2 * 1 Department of Manufacturing Engineering & Engineering Management , City University of Hong Kong, Hong Kong 2 Department of Materials Science and Engineering, Pusan National University, Busan, Korea, * Corresponding author (bizhui@pusan.ac.kr) Keywords : transparent conducting oxide, sol-gel, zinc oxide, nickel oxide Abstract achieved by using indium doped ZnO (IZO) directly on p-GaN [4]. However, the operation voltage of the A single layer composite transparent conducting LED with an IZO TCE is still high. These studies oxide films (NiO:AZO) was successfully synthesized by a simple and low cost sol-gel conclude that serious drawbacks of the current solution method. The results indicated that the TCO thin films for GaN LEDs with high enhancement of conductivity could be achieved by luminous efficacy remain; these limitations annealing the NiO:AZO films in N 2 /H 2 forming gas. include high contact resistance to p-GaN and low P-type conductivity of NiO:AZO film with carrier concentration and mobility. Research on 1.24 10 -3 cm, developing high performance TCO thin films for resistivity of p-type carrier GaN LEDs with high luminous efficacy is still concentration of 4.09 10 20 cm -3 , and a Hall mobility of 12.7 cm 2 /Vs were obtained when molar ratio of limited. To reduce the interface resistance between NiO is 1.5%. This study opens the possibility of TCO and p-GaN, a layer of p-type NiO thin film developing high conductive p-type composite (possessing nickel vacancies and oxygen transparent conducting oxide for optoelectronics interstitials properties) was added between the devices. TCO and p-GaN [5]. This NiO layer was created through thermal annealing of Ni/Au (5 nm/5 nm) 1 Introduction metal in an air ambient on p-GaN. In general, GaN Among the transparent conducting oxide (TCO) LEDs incorporated with TCO thin films as TCEs are materials, indium tin oxide (ITO) is the most fabricated using two separate processes (e-beam typical and extensively used. It has a conductivity evaporation of Ni metal and magnetron sputtering of of 3000–5000 S/cm and transparency of 85–90 % TCO thin films). To simplify the fabrication in the visible range. However, the rising cost of process and resolve the unsatisfied electrical indium [1] as well as unsatisfied ITO electrical performance of current planar AZO thin films in conductivity raises the significant question of GaN LEDs, it is important to develop a desirable whether this material will be continuously used in single layer composite TCO (cTCO) thin film which future optoelectronic devices. Other alternative acts as TCE to p-GaN for GaN LEDs with high TCO candidates, such as aluminum doped ZnO luminous efficacy. This paper investigates the (AZO), and gallium doped ZnO (GZO), suffer structural, electrical and optical properties of a from high contact resistivity with p-GaN layer, single layer cTCO thin film (NiO:AZO) with low carrier concentration and mobility, and low different molar ratios of NiO doped into AZO by optical transmittance compared with ITO sol-gel solution method. materials. In particular, parasitic resistance in the form of contact resistance is one of the main 2 Experimental problems that substantially reduce the overall performance of LEDs. For example, non-ohmic 2.1 Materials contact with a rectifying property results when Zinc acetate dihydrate, aluminum nitrate AZO is deposited directly onto p-GaN [2, 3]. nonahydrate, nickel chloride hexahydrate, propylene Other studies have shown that a very low specific glycol monoethyl ether (PGME) and diethanolamine contact resistance of 3.4 × 10 − 4 cm 2 can be
(DEA) were obtained from Sigma Aldrich. All chemicals were analytic grade reagents and used 3.1 X-ray diffraction without further purification. Fig. 2. shows the XRD patterns of AZO and NiO:AZO thin films with different molar ratios of NiO. All films have mean thickness around 400 nm. 2.2 Synthesis of AZO and NiO doped AZO thin films The AZO film exhibited polycrystalline wurtzite Undoped AZO and NiO doped AZO (NiO:AZO) hexagonal structure matched with JCPDS file no. thin films were prepared by sol-gel method. First, 36-1451. In the 550 C N 2 /H 2 annealed film, the 0.75 M of zinc acetate dihydrate was dissolved in Bragg peaks corresponding to hexagonal ZnO PGME. To extract a sufficient quantity of zinc appear at 2 = 32, 34.4, 35.9, 47.2, 57.5, 62, and acetate dihydrate, equal moles of DEA were dripped 66.7 from the (100), (002), (101), (102), (110), and mixed with PGME. AZO precursor solution was (103) and (112) reflections, respectively. It is prepared by adding the required amount of noteworthy that the (002) peak was weaker than aluminum nitrate nonahydrate to the above solution (100) and (101) peaks. This is because the annealing such that Al/Zn at an atomic ratio of 1.5%. The treatment at 550 C in N 2 /H 2 forming gas made H 2 mixture was then stirred for 1 h at room temperature, combined with O atom in the AZO film, resulting in until a clear, transparent AZO solution was formed. increasing the oxygen defects and lowering the The addition of different molar ratios (0.5 %, 1.0%, crystallinity of the film. As seen, all films containing 1.5%, and 2%) of NiO was added by dissolving NiO doping showed polycrystalline Ni 2 O 3 (202) nickel (II) chloride hexahydrate into the AZO phase at 56.7 . The formation of Ni 2 O 3 crystalline solution. The prepared solution was then stirred phase can be explained by comparing the magnitude overnight. of bond enthalpies between Zn-O (159 kJmol -1 ) and To form the thin film, prepared solution was Ni-O (382 kJmol -1 ) in AZO lattice. The weaker bond dripped onto a clean glass substrate (Corning glass enthalpy of Zn-O helps to provide excess oxygen to E2000). The substrates were spin-coated at 3000 surrounding Ni atoms in AZO matrix, resulting in rpm for 30 s and placed onto a hotplate at a the formation of Ni 3+ ions which subsequently form temperature of 160 °C for 10 min in order to dry the the Ni 2 O 3 phases. The intensity of (202) peak films. After this step, the substrates were put into a increased with molar ratios of NiO from 0.5 to 1.0 400 °C furnace for 10 min to evaporate the solvent mol% in the films. The initial increase in the and remove organic residues. The spin coating and intensity of such peaks may be attributed to the drying procedures were repeated 10 times. Finally, increase in the degree of crystallinity by increasing the thin film was placed into a tube furnace in Ar at the NiO solution molarity [6]. As NiO molar ratios 550 °C for 2 hr to yield the thin film. The sintered increased from 1.5 to 2.0 mol%, the intensity of ZnO samples were finally annealed in tube furnace in and Ni 2 O 3 peaks decreased. This can be explained by N 2 /H 2 (95/5) forming gas at 550 °C for 1 hr to reduce the disturbance of the AZO crystal structure due to NiO to Ni nanoparticle to improve the NiO:AZO the introduction NiO which has cubic phase in the film electrical conductivity. The overall process is NiO:AZO films. In addition, the diffraction angles summarized in a flow diagram in Fig. 1. of the (002) peaks shifted toward low angle with increasing NiO molar ratios. The grain size of these 2.3 Characterization films as estimated by Debye-Scherrer’s formula [7], The thickness of the films was measured by using D = 0.9 / cos (1) surface profiler (Dektak3, VEECO). The where D is diameter of the crystallites forming the crystallinity of the films was estimated by X-ray film, is the wavelength of CuK line (0.154 nm), diffraction (XRD, Bruker AXS D8 Discover) with is the full width at half maximum (FWHM) of the Cu K α (0.154nm). The electrical and optical (002) peak in radians and is the Bragg angle. properties were measured by Hall effect Variation of FWHM and grain size with NiO molar measurement (HMS-3000, ECOPIA) and UV- ratios in NiO:AZO films were shown in Table 1. Visible spectroscopy (HP 8453, Agilent). Except the NiO:AZO film with NiO molar ratio of 0.5, the grain size of NiO:AZO films is increased 3 Results and Discussions
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